Gas turbine engine fan blades, compressor blades and turbine blades are subjected to a combination of low cycle fatigue (LCF) and high cycle fatigue (HCF) stresses during the engine's operation. These LCF and HCF stresses have a detrimental effect on the integrity of the blades. The LCF stresses result from the centripetal force experienced by the blades as they rotate about the engine axis. The HCF stresses result from aerodynamic and other vibration excitation of the blades.
In order to design fan blades, compressor blades and turbine blades that are resistant to fatigue, a good understanding is required of the combination of the steady and alternating stresses to which a blade may be subjected in operation.
The fatigue testing of materials under conditions representative of gas turbine operating conditions is difficult to achieve for blade aerofoil shapes and blade root shapes. Conventional LCF, HCF and fatigue crack growth (FCG) testing on simple specimen shapes have been used to provide mechanical data. Comparisons between these simple specimen shapes and real blades have revealed marked differences in fatigue life. It is known to perform tests on specimens representative of blade shapes, but the behaviour of these specimens does not accurately reflect the behaviour of real blades. Consequently safety factors, typically 50%, are commonly applied to fatigue data.
Dovetail roots are a commonly used method for attaching aero-engine rotor blades to their corresponding disc. They have the advantages of simplicity of manufacture, ease of assembly and high load carrying capability. A dry film lubricant, such as molybdenum disulphide, is commonly applied to the blade and disc dovetail to maintain low friction and prevent damage to the metal surfaces.
During service operation, the rotor blade assembly is subject to a complex loading system, comprising centripetal load, gas load and vibration. Rotation of the fan assembly results in a large load on the dovetail root, due to centripetal acceleration, as the blade tries to pull out of the retention slot. Hoop stresses are generated in the disc rim, as the disc grows under the influence of its own mass and those of the attached blades. In addition, as the fan blade compresses the incoming air, the pressure differential across the blade causes the aerofoil to bend, imparting an additional bending load into the dovetail root. Finally, blade mechanical resonances, and aerodynamic forcing, impose vibratory loading to the dovetail root.
Under the action of these loads, sliding occurs between the blade and disc dovetail, which in combination with the high contact pressures between the two components, can lead to rapid loss of traditional low-friction dry film lubricants. As the coating breaks down, the friction level rises and stress levels in the blade and disc dovetail edge-of-contact region increase. Further wear of the coating leads to metal-to-metal contact between the blade and disc, resulting in heavy frettage and wear of the underlying material. In some cases, interfacial cold welding and plucking can occur, leading to heavy galling and the generation of additional stress concentration features.
These processes, in combination with the applied loads, can lead to the initiation of fatigue cracking at the dovetail edge-of-contact. The underlying component stress field due to centripetal load, gas bending and vibration may then propagate these cracks, resulting in blade or disc failure. In the worst case, where vibration stress levels are high enough to cause crack propagation, failure may occur in a very small number of flights.
Experiments have shown that edge-of-bedding (EOB) fatigue life is critically dependent on the integrity of any anti-frettage coating applied to the blade/disc bedding flanks. Specifically, the friction level on the dovetail contact flank influences fatigue life.
There is therefore a need to provide fatigue data from specimens whose geometry and stress states are more nearly comparable to real blades in order to aid the design of blades resistant to fatigue or to determine more accurately the working life of real blades.